In recent years, with a trend of developing high speed and highly-functional image forming apparatuses, image forming apparatuses of tandem method using intermediate transfer belts have become available in the marketplace. The image forming apparatus using the intermediate transfer belt is usually provided with image forming units configured with image carriers to develop different colors of toner, charging devices, writing devices, developing devices and cleaning devices at a vicinity of the intermediate transfer belt. By the image forming units, a plurality of colors of toner images are transferred and overlapped onto the intermediate transfer belt to form a color image and then collectively transferred onto a sheet at a transfer position. (For example, Patent Document 1: Unexamined Japanese Patent Application Publication No. 2001-201902)
In an image forming apparatus of the tandem method, it is not always preferable that the speed of the intermediate transfer belt is equal to that of a photoconductive drum, and in the Patent document 1, to prevent shedding phenomenon at the time transfer, the difference of the moving speed between the intermediate transfer belt and the photoconductive drum is purposely created.
Also, in the image forming apparatus of the tandem method, a monochrome mode, in which only the black toner is used, may be carried out. In case the monochrome mode is carried out with the speed difference, in the photoconductive drums which do not carry out image forming, there have been problems that abrasion was accelerated and scratches were likely to be created by grazing with the intermediate transfer belt, whereby the life is shortened. To cope with the above problem, in the image forming apparatus of Patent Document 1, in a single color mode, the moving speed of the photoconductive drums which do not carry out image forming are controlled to be equal to the moving speed of the intermediate transfer belt.
An image forming apparatus disclosed in the patent document 2 (Unexamined Japanese Patent Application Publication No. 2005-156776), there is provided a primary transfer roller which biases the intermediate transfer belt onto the photoconductive drum. By retracting the primary transfer roller, the intermediate transfer belt can be displaced from a position where the belt is biased onto the photoconductive drum to a released position. In the monochrome mode, by retracting the primary roller to bias the photoconductive drum which is not involved in image forming, extension of lives of the photoconductive drums and developing devices which are not used for image forming is realized.    Patent Document 1: Unexamined Japanese Patent Application Publication No. 2001-201902    Patent Document 2: Unexamined Japanese Patent Application Publication No. 2005-156776
In the image forming apparatus of the tandem method, positional accuracy among the toner images formed by individual image forming units is important since an insufficient positional accuracy causes color shift among the toner images.
To ensure the positional accuracy, it is important that rotation of the intermediate transfer belt is controlled with high accuracy, and the moving speed of the photoconductive drum is slower than that of the intermediate transfer belt in some cases for the following reasons:
In case a load of a drive system of the intermediate transfer belt is fluctuated, the rotation speed becomes unstable due to effects of elastic deformation of the drive system and play of gears engaged. If the belt rotates with a load more than a prescribed load, the above effect can be reduced, thus in order to rotate with high accuracy, it is preferred to drive rotation of the belt while applying the load more than the prescribed load.
As a device to apply the load, a brake disposed at the drive system of the intermediate transfer belt can be considered. However because of a complicated mechanism and difficulty of applying the prescribed load for a long period of time, it is not a practical. Whereby it is considered that by making the moving speed of the photoconductive drum slower, the load is applied from the photoconductive drum to the intermediate transfer belt.
However, in the image forming apparatus described in Patent Document 2, in the monochrome mode, number of the photoconductive drums in contact with the intermediate transfer belt is one fourth of that in the full color mode. Thus there is concerned a problem that the load applied to the intermediate transfer belt is extremely reduced and an effect of load torque generated when the sheet passes through the secondary transfer section becomes large, whereby rotation of the intermediate transfer belt cannot be controlled with high accuracy.
Also, the image forming apparatus described in Patent Document 1, the moving speed of the photoconductive drum is changed for the monochrome mode and for the color mode, however since the speed is changed so as to match the moving speed of the photoconductive drum matches to that of the intermediate transfer belt, there is concern that the load of the intermediate transfer belt reduces. Also, since there is no mechanism to release the photoconductive drum, there is a concern that an effect to extend the life is not sufficient.